For use in switching a call connection between subscriber telephone sets via a switching matrix, which interface circuit is provided with two terminals for connection, optionally via a cable, to a telephone set and with two terminals for connection to one side of the switching matrix. The resultant connection can be switched on and off and is typically used for transmitting speech signals. Such a telephone line interface circuit is known from practical engineering.
For DC decoupling the subscriber telephone set from the switching matrix, large capacitors are used in such circuits in order to make a difference possible in voltage level between the circuits on both sides. As a result, such a circuit cannot be constructed in integrated form.